What is the best way to sense low liquid level in containment tanks or overflow pans that are normally not supposed to have any liquid in them?  

INTERSTITIAL FLOATS
These are an option that has the advantage that they lay flat  on the bottom of a vessel.  This makes them easy to deploy by simply dropping them to the bottom of a containment vessel by lowering them by the wire until they rest on the bottom.  When the liquid level rises, the floating element floats up, which triggers the alarm device connected. 

The problem is that they have a substantial thickness to them.  This popular one has a thickness of 9.5 mm.  The liquid must rise substantially higher than that to actuate the switch.

CONDUCTIVITY
Conductivity switches consist of metal elements that are connected by liquid and sensed by a circuit.  Often the format is interdigitated circuits and when a droplet of water  bridges the metal poles of the circuit – an alarm is given.

A problem with this method is that it consists of a metallic circuit whih is subject to corrosion.  Also these need a steady supply of power so its not suitable for battery power.  It will detect the smallest amount of liquid which may be too small – just one drop of condensation will create a false alarm. 

CAPACITIVE SENSOR
A capacitive sensor senses a layer of water at some distance away from the sensor “face”.  Typical distances would be 2-8mm – larger diameter sesnors = longer distance.  The sensor would be oriented vertically so the “face” it pointing down towards the tank bottom.  So the sensor detects any liquid in the bottom without detecting the tank bottom.  For plastic walled tanks this shouldn’t present a problem because most plastics don’t register with these sensors.  Still, capacitive sensors have calibrations that will probably need to be set to detect properly.  It also requires constant power to generate a detecting field in order to sense the liquid.

VERTICAL FLOAT SWITCH
A standard float switch must be buoyed by the liquid. The standard polypropylene float we use (‘model ‘F’) requires about 25 millimeters of liquid to detect a liquid level. 
For sensing very low liquid levels, there are float switches made just for this purpose known as “ultra low level” float switches.  These will float in a liquid level as low as 2 mm depth!  With both float switches, the challenge is to fixture the float switch so it is at the correct depth in a tank.  Gizmo Engineering mounts float switches on a pipe which is set into the tank at the depth needed.  For very low level sensing we would take advantage of our  proprietary “float guards” that surround the float switch so the pipe can be plunged into the tank without fear of damaging the float switch. That way any depth of interstitial liquid from 0.2mm will be reliably detected.  The float offers the advantage of allowing batter operation.